Problems encountered in measuring natural gas using chromatography

It is best to first use a standard gas to check for issues with the chromatography system. Also, your chromatography conditions might be a bit off; the column temperature is too high, which results in poor resolution.

Friend upstairs, what temperature do you usually use for the column temperature?

It’s the first time I’ve seen MPA used as a unit for pressure in nitrogen sources; surely they’re not installing a pipeline pressure gauge on the chromatograph! That company’s chromatography? Excessive! Well, I now have a rough idea of your requirements. For this kind of thing, it’s best to use a spectrum – comparing it with a spectrum makes things much clearer. With the parameters you’ve provided, only rough adjustments can be made; precise adjustments aren’t possible with that device. If you can carry out analysis using this chromatography method, then you can become what’s known as a master at this field! Fortunately, it’s all high-content analysis, so a small difference doesn’t matter. It is recommended to lower the column temperature first and increase the detector temperature by 10 degrees.

There is also the issue of correction factors: this correction factor can be used for the first time when using chromatography, but for the second time, one should have their own correction factor. This type of instrument needs to be calibrated at least once every three months, or whenever its conditions change; it certainly can’t be used for several years without the columns being deactivated and the instrument calibrated

Is the column temperature set incorrectly? Why aren’t the peaks separated? The column temperature is too high

GB/T 13610-2003 Analysis of the composition of natural gas – Gas chromatography method 1. Scope This standard specifies the analytical methods for determining the chemical composition of natural gas and similar gas mixtures using gas chromatography. This standard is applicable to the analysis of natural gas composition within the range shown in Table 1, as well as to the determination of one or several components. This standard does not address all safety issues related to its application. Before using this standard, it is the user’s responsibility to establish appropriate safety and health operating procedures and to define the scope of application thereof. Table 1 Components of natural gas and their concentration ranges (mole fraction)
Component Concentration Range (Mole Fraction to %)
Nitrogen: 0.01–10
Hydrogen: 0.01–10
Oxygen: 0.01–20
Nitrogen: 0.01–100
Carbon dioxide: 0.01–100
Methane: 0.01–100
Ethane: 0.01–100
Propane: 0.01–100
Isobutane: 0.01–10
n-Butane: 0.01–10
Neopentane: 0.01–2
Isopentane: 0.01–2
n-Pentane: 0.01–2
Hexane: 0.01–2
Heptane and heavier components: 0.01–1
Hydrogen sulfide: 0.3–30

2. Normative reference documents
The provisions in the following documents become part of this standard through reference to them. For any referenced document that is dated, all subsequent amendment sheets (excluding corrections) or revised versions are not applicable to this standard; however, parties reaching an agreement under this standard are encouraged to consider whether the latest versions of such documents can be used. For reference documents without a date, the latest version applies to this standard. GB/T5274 Method for the preparation of calibration gas mixtures for gas analysis – Gravimetric method (GB/T5274-1985, ISO6142:1981, EQV) 3. Summary of the method: Representative gas samples and standard mixtures with known compositions (hereinafter referred to as standard gases) are separated using gas chromatography under the same operating conditions. Many heavy-tailed components in the sample can, at a certain time, produce a set of irregular peaks by changing the direction of the carrier gas flowing through the column; these heavy-tailed components can be C. And the more reorganized component, C. and heavier components, or C and heavier components. The corresponding composition of the sample is calculated from the composition values of the standard gas by comparing peak heights, peak areas, or both. 4. Reagents and Materials 4.1 Carrier gas a) Helium or hydrogen, with a volume fraction of not less than 99.99%; b) Nitrogen or hydrogen, with a volume fraction of not less than 9.99%. 4.2 Standard gases The standard gases required for analysis can be secondary reference materials, or they can be prepared in accordance with GB/T5274. In the analysis of oxygen and nitrogen components, diluted dry air is a suitable standard. All components of the standard gas must be in a homogeneous gaseous state. For components with a mole fraction of no more than 5%, the mole fraction of the corresponding component in the standard gas should be no more than 10% compared to that in the sample, and it should also be no less than half of the concentration of the corresponding component in the sample. For components with a mole fraction greater than 5%, the concentration of the corresponding component in the standard gas should be no less than half of the concentration of that component in the sample, and no more than twice its concentration. 5 Instruments and Equipment 5.1 Detector A thermal conductivity detector should be used, or a detector with comparable sensitivity and stability. It is required that for a gas sample with a butane molar fraction of 1%, injecting 0.25 mL should generate a signal of at least 0.5 mV. 5.2 The recording system may use a recorder, an electronic integrator, or a microprocessor. 5.2.1 Recorder: The full scale of the recorder is 1 mV to 5 mV; the width of the recording paper shall be at least 150 mm, and the maximum response time of the recording pen shall be equal to or less than 2 s. If chromatographic peaks are measured manually, a faster paper speed is required; this speed can reach up to 10 mm/min. 5.2.2 Electronic integrators or microcomputers can detect the chromatographic separation and record the response values. 5.3 Attenuator: When manually measuring chromatographic peaks, an attenuator must be used to keep the maximum peak value of the detector’s output signal within the range covered by the recorder’s paper. The error between different attenuation settings must be less than 0.5%. 5.4 The sampling system should be made of materials that are inert to and do not adsorb the components present in the gas sample; stainless steel is the preferred material for this purpose. The injection system should be equipped with an injection valve featuring a dosing tube, the volume of which is . .25m1,-2ml, inner diameter 2mm; those smaller than 2mm should be equipped with a heater. For sampling under vacuum, the piping arrangement shown in Figure 1 can be used. For vacuum pumps, according to GB/T 13610-2003, section 5.5 on column temperature control: during constant-temperature operation, the column temperature remains constant, with any variations being within a specified range. Within 0.3℃, during programmed temperature rise, the column temperature should not exceed the recommended temperature limit for the filler in the column. 5.6 Detector temperature control: Throughout the analysis, the detector temperature should be equal to or higher than the highest column temperature, and it must remain constant, with any variation being within 0.3°C. 5.7 Carrier gas control: Throughout the analysis, the flow rate of the carrier gas remains constant, with any variation being within 1%. 58 The material of the chromatography column must be inert and non-absorptive toward the components in the gas sample; stainless steel tubes should be given priority. The filler inside the column should be able to achieve the specified requirements for the separation of the components being analyzed. 5.8.1 The adsorption column must be capable of completely separating oxygen, nitrogen, and methane; the separation factor R must be greater than or equal to 1.5, with this separation factor being calculated using Equation (1). Figure 2 is a typical chromatogram obtained using an adsorption column. R = 2 × (t1 – t2) / W, where: t1 is the absolute retention time of the first chromatographic peak, in seconds; t2 is the absolute retention time of the second chromatographic peak, in seconds; W is the peak width of the first chromatographic peak, in seconds; and W2 is the peak width of the second chromatographic peak, in seconds. Units: minutes. Chromatography conditions: chromatographic column, 1X molecular sieve, column volume of 0.2^0.1) mm³; pressure levels of 5, 2 kPa, 6, 5 kPa, 7, 8 kPa, and 91 kPa. Data should be recorded in accordance with GB/T 13610-2003, section 6.1.1.2 regarding linearity checks. Notes: a) At atmospheric pressure, the compressibility of nitrogen, methane, and ethane is less than 10^-a. Other components in natural gas still exhibit significant compressibility at pressures below atmospheric pressure. b) For components with a vapor pressure of less than 100 kPa, their linearity cannot be determined using pure gases due to insufficient vapor pressure. For such components, nitrogen or methane can be mixed with them to obtain their partial pressures, thereby bringing the total pressure to 10 kPa. The saturated vapor pressures of common components in natural gas at 38°C are shown in Table 4. c) A standard gas containing various components to be measured can be used, and linear verification can be carried out by taking samples at different pressures. Table 4 Vapor pressures of various components in natural gas at 38°C Component Absolute pressure/kPa N, >34500 CH, >34500 CO, > 5520 C,H, >5 520 H,S 2 720 C,H, 1 300 iC4H,o 501 nC4H,a 356 iC,H 141 nC;H 108 nC,H 34.2 nC,H. 11.2 6.2 Instrument repeatability check: Once the instrument is stable, standard gas tests should be conducted twice or more in succession, with the difference in response values for each component having to be within 1%. Under unchanged operating conditions, whether it is two consecutive injections or the last injection compared to one of the previous injections, as long as the difference for each component is within 1%, it can be used as a standard for subsequent gas sample analysis; it is recommended to perform calibration operations on a daily basis. 6.3 Preparation of the gas sample: If it is necessary to remove hydrogen sulfide, two methods are available (see Appendix B). In the laboratory, the sample must reach equilibrium at a temperature that is 10–20°C higher than the temperature of the gas source at the time of sampling. The higher the temperature, the shorter the time (for sample containers of 30 mL or less, it takes about 2 hours). This method assumes that the liquid entrained in the gas has been removed during on-site sampling. If the gas source temperature is higher than the laboratory temperature, the gas sample must be preheated before entering the chromatograph. If it is known that the hydrocarbon dew point of the gas sample is below the lowest ambient temperature, heating is not required. 6.4 Sampling: To obtain a linear response from the detector for each component, especially methane, the sampling volume should not exceed a certain limit. .5mL。 Except for trace components, sufficient precision can be achieved using such injection volumes. When determining components with a mole fraction of 5% or less, the injection volume may be increased to 5 mL. The connection tubing between the sample vial and the instrument’s injection port should be made of stainless steel or polytetrafluoroethylene; copper, polyethylene, polyvinyl chloride, or rubber tubing must not be used. 6.4.1 Purging method: Open the outlet valve of the sample vial and purge the sampling system, including the volumetric tube, with gas sample. For each instrument, the required purge volume must be determined and verified. The sampling pressure in the dosing tube should be close to atmospheric pressure; the valve of the sample bottle should be closed to stabilize the gas pressure in the dosing tube. Then, the gas sample in the volumetric tube is immediately introduced into the chromatography column to prevent the infiltration of contaminants. 9 6.4.2 Liquid-sealing replacement method: If the gas sample is obtained using the liquid-sealing replacement method, the gas sample in the liquid-sealing replacement bottle can be used to purge the sampling system, including the volumetric tube. Certain components, such as carbon dioxide, hydrogen sulfide, hexane, and heavier components, may be partially or completely removed by water or other sealing fluids; the sealing fluid replacement method should not be used when precise measurements are required. 6.4.3 Vacuum method: Evacuate the sampling system to an absolute pressure of less than 10 Pa, close the valve connected to the vacuum system, then carefully fill the gas sample from the sample bottle into the volumetric tube to the desired pressure, and subsequently introduce the gas sample into the chromatography column. 6.5 Operation of the distribution column for separating ethane from heavier components and carbon dioxide: Helium or hydrogen is used as the carrier gas, an appropriate injection volume is selected for injection, and the heavier components are purged at the right time. The response corresponding to the standard gas is obtained in the same manner. If this chromatography column can separate methane from nitrogen and oxygen (see Figure 4), it can also be used to determine methane levels, provided that the injection volume does not exceed 0.5 mL. For separating oxygen, nitrogen, and methane using this adsorption column, helium or hydrogen is used as the carrier gas; for methane analysis, the injection volume must not exceed 0.5 mL in order to obtain responses for oxygen, nitrogen, and methane in the gas sample. The responses of the nitrogen and methane standard gases were obtained in the same manner. If necessary, dry air under a certain vacuum pressure with the pressure measured precisely, or dry air diluted with helium, is used to obtain the responses of oxygen and nitrogen. Note: A mixture with an oxygen content of about 1% can be prepared by filling a cylinder of dry air at atmospheric pressure with helium to a pressure of 2 MPe; this pressure does not require precise measurement. Because the nitrogen in this mixture must be determined by comparing it with the nitrogen in the standard gas. The molar fraction of nitrogen in this mixture is multiplied by. .268 is the molar fraction of oxygen, or multiplied by it. .280 is the mole fraction of oxygen plus wings; the oxygen standard gas prepared a few days ago is unreliable. Since the response factor for oxygen is relatively stable, a response factor of 6.7 can be used for oxygen. For the operation of the adsorption column used to separate helium and hydrogen, nitrogen or hydrogen is used as the carrier gas, with a sample volume of 1 mL–5 mL. The responses of helium and hydrogen were recorded, and the corresponding responses of standard gases of appropriate concentrations of helium and hydrogen were obtained using the same method (see Figure 7). (mw) Chromatographic conditions: Chromatography column: 13X molecular sieve; column length: 2 m; column temperature: 50°C; detector current: 10 mA; carrier gas: nitrogen. 1 – Helium; 2 – Hydrogen. Figure 7 Typical chromatograms of the separation basin and inner container. 7 Calculations. 7.1 Selection of data: The significant figures of the concentration of each component should be determined based on the precision of the measuring instrument and those of the standard gas. The number of significant figures in the concentration of any component in the gas sample should not be more than that in the concentration of the corresponding component in the standard gas. 7.2 External standard method 7.2.1 Measurement of pentane and lighter components: The peak height or peak area of each component is measured, and the responses of the corresponding components in the gas sample and the standard gas are adjusted to the same attenuation level. The concentration Y of a given component in the gas sample, as specified in GB/T 13610-203, is calculated using equation (2): Y = Ys · … · ..In equation (2): Y is the molar fraction of component i in the standard gas; H is the peak height or peak area of component i in the gas sample; H’ is the peak height or peak area of component i in the standard gas. H and H’ are expressed in the same units. If air is introduced as a standard gas for oxygen or nitrogen under a certain vacuum pressure, pressure correction is performed according to equation (3). Y = y(H,/H)(p,1p,) · .. (3) Where: P is the absolute pressure during air injection, in kPa; Pb is the actual atmospheric pressure during air injection, in kPa. 7.2.2 For the measurement of hexane and heavier components, the peak areas of hexane, heptane, and heavier compounds resulting from backflushing are measured, while the peak areas of n-pentane and isopentane are also measured on the same chromatogram. All measured peak areas are converted to the same scale; details on the correction method are provided in Appendix B, and information regarding the arrangement of the chromatography columns is given in Appendix C. The concentrations of hexane (C) and heptane (C`) in the gas sample are calculated using equation ( ). Y(C,)=y(C)A(C)M(C,), ... , .·. ·...(4) Where: y(C,) is the mole fraction of the component with carbon count n in the gas sample, expressed as a percentage; Y(C,) is the sum of the mole fractions of isopentane and n-pentane in the gas sample, also expressed as a percentage; A(C,) is the peak area of the component with carbon count n in the gas sample; A(CS) is the sum of the peak areas of isopentane and n-pentane in the gas sample. Both A(C) and A(C,) are expressed in the same units. M(C,) is the relative molecular mass of pentane, with a value of 72; M(C) is the relative molecular mass of a compound with n carbon atoms – for C6, it is 86, and for other compounds it represents the average relative molecular mass. If the concentrations of isopentane and n-pentane have already been determined separately using smaller injection volumes, there is no need to determine them again. 7.2.3 Normalization: The normalized molar fraction of each component is obtained by multiplying the original content value of that component by 100 and then dividing by the sum of the original content values of all components. The difference between the sum of the original content values of all components and 10.0% should not exceed 1.0 ring. An example of the calculation for gas samples is provided in Appendix Do. 8 Precision: The following criteria are used to determine whether the measurement results are reliable. 8.1 Repeatability: The results obtained from repeated analyses of the same gas sample using the same instrument by the same operator are considered suspicious if the difference between two consecutive measurements exceeds the value specified in Table 5. 8.2 Reproducibility: For the analysis results of the same gas sample provided by two different laboratories, if the difference between those results exceeds the value specified in Table 5, the results from each laboratory should be considered suspicious. Encapsulation: 5 precise composition concentration ranges (mole fraction); repeatability and reproducibility: 0–0.1, 0.01, 0.02, 0.1^, 1.0, 0.04, 0.07, 1.0, 5.0}; 0.07 – 5.0–10 –; 0.12 > 10: 0.20, 0.30

I’m sorry, you can look up the standard by yourself!